Applied in industrial process-measurements technology, especially also in connection with the automation of chemical processes or procedures for producing a product from a raw or starting material by use of chemical, physical or biological processes and/or the automated control of industrial plants, are so-called field devices, namely electrical measuring- and/or switching devices installed directly at the respective plant, i.e. devices such as e.g. Coriolis mass flow measuring devices, density measuring devices, magneto inductive flow measuring devices, vortex flow measuring devices, ultrasonic, flow measuring devices, thermal mass flow measuring devices, pressure measuring devices, fill level measuring devices, fill level limit switches, temperature measuring devices, pH-value measuring devices, etc., which serve for producing analog or, for example, also digital, measured values representing process measurement variables as well as measured value signals lastly carrying these values. The respective process measurement variables to be registered include, depending on application, for example, mass flow, density, viscosity, fill level or limit level, pressure or temperature or the like, of a liquid, powdered, vaporous or gaseous medium conveyed, respectively held, in a corresponding container, such as e.g. a pipeline or a tank.
For registering the respective process measurement variable and for transducing the same into an electrical measurement signal corresponding therewith, field devices have, in each case, a corresponding physical to electrical or chemical to electrical, measuring transducer, which is applied most often directly in a wall of the container holding the medium or in the course of a line, for example, a pipeline, conveying the medium. For processing the measurement signal, the measuring transducer is further connected with an electronics of the field device serving for further processing or evaluation of the at least one measurement signal, as well as also generating corresponding measured values. The electronics is typically electrically connected via corresponding connecting lines to an external supply circuit, for example, a measurement transmitter feed unit, a power isolator or a power supply, from which the electronics is supplied with electrical energy during operation. Examples of such measuring systems known per se to those skilled in the art are disclosed in, among others, Published International Applications WO-A 88/02853, WO-A 88/02476, U.S. Pat. No. 6,452,493, US-A 2011/0317390, the European Patents EP-A 816 807, and EP-A 1 591 977 or US-A 2010/0101817, respectively are buyable from the applicant, for example, under the marks, FLOWPHANT®T DTT31, t-switch ATT11, t-trend ATT12, Magphant DTI200, Promag 53H, Prowirl 73F, Promass 83X, or Promass 84F.
The device electronics of such field devices are most often accommodated in a comparatively robust, for instance, impact-, pressure-, explosion- and/or weather resistant, electronics housing. The electronics housing can be arranged e.g. removed from the field device and connected with the field device only via a flexible cable; it can, however, also be arranged directly on the measuring transducer or on a measuring transducer housing separately housing the measuring transducer. For display of measured values on-site, such electronics have, furthermore, most often also a display element placed in such electronics housing, for example, an LCD display element, which in modern field devices can, for example, be formed also by means of a combined display/interaction unit, which can, in given cases, also be removable.
In the case of a large number of field devices with measuring transducer used in industrial measurements technology, the measuring transducer is, for producing the measurement signal during operation, additionally so operated by a driver signal generated at least at times by the operating- and evaluating circuit that the measuring transducer acts at least indirectly on the medium, or, however, also practically directly via a probe directly contacting the medium, in a manner suitable for measuring, in order to bring about reactions there corresponding with the process measurement variable respectively to be registered and reacting on the measuring transducer. The driver signal can, in such case, be correspondingly controlled, for example, as regards an electrical current level, a voltage level and/or a frequency. Examples of such active measuring transducers, thus measuring transducers correspondingly converting an electrical driver signal in the medium, include, especially, flow measuring transducers serving for measuring media flowing, at least at times, e.g. flow measuring transducers having at least one magnetic field producing coil driven by the driver signal, or at least one ultrasonic transmitter driven by the driver signal, or, however, also fill level- and/or limit level transducers serving for measuring and/or monitoring fill levels in a container, such as e.g. ones with freely radiating microwave antennas, with a Goubau line or with vibrating immersion element.
The measured values generated by the device electronics are typically provided to corresponding circuit outputs in the form of electrical, digital or analog, measured value signals, for example, in the form of an analog signal current correspondingly modulated in the range from 4-20 mA. Moreover, usual for providing measured values are also so-called frequency outputs, namely circuit outputs encoding the measured values in a pulse sequence frequency of a binary rectangular signal, or also so-called pulse outputs, namely circuit outputs signaling the reaching of an earlier selectable, quantized unit in the form of a pulse. Besides field devices serving principally for registering a physical, measured variable and having, in each case, a measuring transducer, there are additionally field devices, embodied, for example, as a electric motor driven actuator of valves or pumps. These field devices are provided, especially, to set one or more physical process parameters, consequently to engage actively in the respective process, supplementally, however, also to provide internal measuring—and/or setting values on corresponding circuit outputs. Besides circuit outputs of the aforementioned type providing measured values, the device electronics of a modern field device can, at times, also have so-called status—, respectively alarm, outputs and/or also circuit outputs serving for the immediate activating of external electrical devices directly connected to the particular field device, and, consequently acting as relays. Such circuit outputs can be implemented as passive circuit outputs, namely circuit outputs coupled into an electrical circuit driven by a voltage source located externally of the field devices or also as an active circuit output coupled into an electrical circuit driven by a voltage source internal to the field devices. Alternatively or supplementally, field devices can also have circuit outputs for connecting to a fieldbus, for example, a serial fieldbus.
For connecting internal electrical, respectively electronic, circuits, namely electrical, respectively electronic, circuits accommodated within the electronics housing, not least of all also circuit outputs of the above mentioned type, with one or more external electrical circuits, for example, also the mentioned external supply circuits, field devices have, in each case, a connection apparatus for the electrical connecting of a two or multiline connection cable leading to the field device, consequently a connection cable running at least partially externally of the respective electronics housing. Such connecting apparatuses of field devices are sometimes formed by means of a plug connector, of which a first plug connector part is electrically connected to the particular circuit of the field device and at the same time is secured to a platform placed within the electronics housing, for example, an electronics insert or a circuit board, and a second plug connector part complementary to the first plug connector part is connected with the connection cable. In operation of the respective field device, the plug connector parts are connected fixedly as well as releasably with one another to form an electrical current path leading from the connection cable to the device electronics. In the case of plug connectors of the type being discussed, the first plug connector part is most often embodied as an installed plug and the second plug connector part correspondingly as a socket, in such a manner that the first plug connector part has at least one contact pin electrically connected to the circuit of the field device and the second plug connector part at least one contact socket electrically connected with at least one conductor of the connection cable, and that the two plug connector parts are so connected with one another that the at least one contact socket of the second plug connector part is plugged onto the at least one contact pin of the first plug connector part to form a mechanical connection based on a frictional interlocking and at the same time electrically well conductively contacts the contact pin as a result of an areal pressure produced between contact socket and contact pin to form an electrical connection in the form of a plug contact. By application of such plug connectors, the electrical connecting of a field device to external electrical circuits, respectively the respective connecting lines, is significantly simplified and, at the same time, the failure susceptibility of the electrical current path typically first established on-site by means of the respective plug connector can be kept very small, for example, also by embodying the plug connector according to the known poka yoke principle. Conversely, an electrical current path formed by means of such a plug connector can also be very quickly interrupted by withdrawing the at least one contact socket of the second plug connector part from the contact pin of the first plug connector part along a predetermined track, in order to separate the second plug connector part from the first plug connector part using a removal force acting in the direction of the track.
A disadvantage of such plug connectors is, however, that they, not least of all because of the way they function, develop most often only very low holding forces, respectively that, conversely, just a low removal force acting on the second plug connector part can bring about a separating of the second plug connector part from the first plug connector part. This can, for example, also lead to the fact that such plug connectors can, from vibrations of the electronics housing from whatever source, for instance, caused by shaking movements of the respective container transferred to the field device, automatically loosen, with this leading to a sudden failure of the field device due to interrupted electrical current paths. There exist, consequently, also corresponding specifications for device safety of such connecting apparatuses of field devices conforming, for example, also to one of the established standards C22.2 No 213-M1987, ANSI/ISA-12.12.01-2010, FM3611-2004, respectively IEC 60079-15:2010, according to which in a field device for separating the two plug connector parts of a plug connector of the type being discussed a removal force of at least 15 N (Newton) must be offered, respectively, conversely, with removal forces of less than 15 N acting on the second plug connector part no separating of the plug connector, consequently no removal of the contact socket of the second plug connector part from the at least one contact pin of the first plug connector part is allowed to happen. Moreover, however, also holding forces exerted by such plug connectors and having originally sufficient magnitude can in the course of time, not least of all as a result of material fatigue, gradually lessen.